Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is a static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to a high voltage electrical power system and a DC side connected to a temporary electric power storage means such as capacitors. The STATCOM can supply reactive power to or absorb reactive power from the transmission line.
As opposed to the STATCOM, which only compensate for reactive power, another concept is to connect a dc power source to a compact STATCOM, and thus being able to perform active power compensation. The construction may be used e.g. as a spinning reserve and for compensating for fluctuating energy levels in the power system.
Today the dc power source is a high voltage battery. Since the power apparatus is connected to the ac voltage of the high voltage electrical power system, a large number of battery cells have to be connected in series to match the dc voltage of the power apparatus. Moreover, to obtain a desired amount of electric energy (duration of active power) of the energy storage, a number of strings including a plurality of battery cells could be connected in parallel. The first conductor and the batteries themselves are typically placed at a distance to the second conductor. Since the conductors often consists of non-isolated metal bus bars this safe distance is important to prevent an accidental and contemporaneous contact of the conductors.
Furthermore, in case of a fault, such as a short-circuit, the power apparatus has to be protected. Consequently, two switches are provided in each string to disconnect the string in case the converter, a string, or a part of the string is short-circuited. One switch is able to disconnect the string from the positive dc rail of the converter and the other switch is able to disconnect the string from the negative dc rail of the converter. Each of the switches has to be rated for the entire converter voltage to be capable of protecting the power apparatus.
Up to now, mechanical dc circuit breakers have been used as switches for connection and disconnection of the strings. However, the availability of dc circuit breakers is limited and the ratings of these breakers are considerably low and less than the required voltage ratings in high voltage electrical power systems. To overcome this problem mechanical ac circuit breakers with some additional circuitry, such as a resonance circuit connected in parallel with the ac circuit breaker, have been used.
A solid-state switch is an electronic switch that, unlike a mechanical breaker, contains no moving parts. There is a desire on the market to be able to exchange the mechanical ac circuit breakers with solid-state switches as proposed in WO/2009/152849 where solid-state switches are distributed among the dc power-source units. However, high voltage power applications require large electric bus bar constructions that inevitably lead to high loop inductance. The loop inductance acts as a limiter of the current derivative (di/dt) during switching transitions. Since the turn-off of solid-state switches is much faster than the turn-off of mechanical breakers, large loop inductance implies high energy storage in the electric bus bar constructions and during fast interruption this energy will appear as a voltage spike across the switch (see equation 1 & 2). This voltage spike will have fatal effects and destroy the switches.
                              E          L                =                              1            2                    ⁢                      LI            2                                              (        1        )                                          V          switch                =                  L          ·                                    ⅆ              i                                      ⅆ              t                                                          (        2        )            
A known solution to reduce high voltage spikes across circuit breakers is to use additional equipment such as different kinds of snubber circuits and varistors connected to the breaker. Such additional equipment is however costly.